This invention relates in general to coatings that emit magnetic resonance signals and in particular, to such coatings containing paramagnetic metal ions, and to a process for coating devices and implants with such coatings so that these devices are readily visualized in magnetic resonance images during diagnostic or therapeutic procedures done in conjunction with magnetic resonance imaging (MRI).
Since its introduction, magnetic resonance (MR) has been used to a large extent solely for diagnostic applications. Recent advancements in magnetic resonance imaging now make it possible to replace many diagnostic examinations previously performed with x-ray imaging with MR techniques. For example, the accepted standard for diagnostic assessment of patients with vascular disease was, until quite recently, x-ray angiography. Today, MR angiographic techniques are increasingly being used for diagnostic evaluation of these patients. In some specific instances such as evaluation of patients suspected of having atheroscleroic disease of the carotid arteries, the quality of MR angiograms, particularly if they are done in conjunction with contrast-enhancement, reaches the diagnostic standards previously set by x-ray angiography.
More recently, advances in MR hardware and imaging sequences have begun to permit the use of MR for monitoring and control of certain therapeutic procedures. That is, certain therapeutic procedures or therapies are performed using MR imaging for monitoring and control. In such instances, the instruments, devices or agents used for the procedure and/or implanted during the procedure are visualized using MR rather than with x-ray fluoroscopy or angiography. The use of MR in this manner of image-guided therapy is often referred to as interventional magnetic resonance (interventional MR). These early applications have included monitoring ultrasound and laser ablations of tumors, guiding the placement of biopsy needles, and monitoring the operative removal of tumors.
Of particular interest is the potential of using interventional MR for the monitoring and control of endovascular therapy. Endovascular therapy refers to a general class of minimally-invasive interventional (or surgical) techniques which are used to treat a variety of diseases such as vascular disease and tumors. Unlike conventional open surgical techniques, endovascular therapies utilize the vascular system to access and treat the disease. For such a procedure, the vascular system is accessed by way of a peripheral artery or vein such as the common femoral vein or artery. Typically, a small incision is made in the groin and either the common femoral artery or vein is punctured. An access sheath is then inserted and through the sheath a catheter is introduced and advanced over a guide-wire to the area of interest. These maneuvers are monitored and controlled using x-ray fluoroscopy and angiography. Once the catheter is properly situated, the guide-wire is removed from the catheter lumen, and either a therapeutic device (e.g., balloon, stent, coil) is inserted with the appropriate delivery device, or an agent (e.g., embolizing agent, anti-vasospasm agent) is injected through the catheter. In either instance, the catheter functions as a conduit and ensures the accurate and localized delivery of the therapeutic device or agent to the region of interest. After the treatment is completed, its delivery system is withdrawn, i.e., the catheter is withdrawn, the sheath removed and the incision closed. The duration of an average endovascular procedure is about 3 hours, although difficult cases may take more than 8 hours. Traditionally, such procedures have been performed under x-ray fluoroscopic guidance.
Performing these procedures under MR-guidance provides a number of advantages. Safety issues are associated with the relatively large dosages of ionizing radiation required for x-ray fluoroscopy and angiographic guidance. While radiation risk to the patient is of somewhat less concern (since it is more than offset by the potential benefit of the procedure), exposure to the interventional staff can be a major problem. In addition, the adverse reactions associated with MR contrast agents is considerably less than that associated with the iodinated contrast agents used for x-ray guided procedures.
Other advantages of MR-guided procedures include the ability to acquire three-dimensional images. In contrast, most x-ray angiography systems can only acquire a series of two-dimensional projection images. MR has clear advantages when multiple projections or volume reformatting are required in order to understand the treatment of complex three-dimensional vascular abnormalities, such as arterial-venous malformations (AVMs) and aneurysms. Furthermore, MR is sensitive to measurement of a variety of “functional” parameters including temperature, blood flow, tissue perfusion, diffusion, and brain activation. This additional diagnostic information, which, in principle, can be obtained before, during and immediately after therapy, cannot be acquired by x-ray fluoroscopy alone. It is likely that once suitable MR-based endovascular procedures have been developed, the next challenge will be to integrate this functional information with conventional anatomical imaging and device tracking.
Currently, both “active” and “passive” approaches are being used for visualization and monitoring of the placement of devices and materials used for therapeutic procedures done using MR guidance. When active tracking is used, visualization is accomplished by incorporating one or more small radio-frequency (RF) coils into the device, e.g., a catheter.
The position of the device is computed from MR signals generated by these coils and detected by MR imager. This information is superimposed on an anatomical “road map” image of the area in which the device is being used. The advantages of active tracking include excellent temporal and spatial resolution. However, active methods allow visualization of only a discrete point(s) on the device. Typically, only the tip of the device is “active”, i.e., visualized. Although it is possible to incorporate multiple RF coils (4-6 on typical clinical MR systems) into a device, it is still impossible to determine position at more than a few discrete points along the device. While this may be acceptable for tracking rigid biopsy needles, this is a significant limitation for tracking flexible devices such as those used in endovascular therapy. Furthermore, intravascular heating due to RF-induced currents is a concern with active methods.
The attachment of coils onto flexible catheters presents numerous challenges in maintaining the functionality of the catheter as these coils result in changes in the mechanical properties of the catheter onto which they are incorporated. Ladd et al. [Ladd et al., Proc. ISMRM (1997) 1937] have addressed some of the deficiencies of an active catheter by designing a RF coil that wraps about the catheter.
This allows visualization of a considerable length of a catheter, but still does not address the problems of RF heating and the mechanical changes which degrade catheter performance.
One technique for passive tracking is based on the fact that some devices do not emit a detectable MR signal and also cause no artifacts in the MR image. This results in such a device being seen as an area of signal loss or signal void in the MR images. By tracking or following the signal void, the position and motion of such a device can be determined. One advantage of passive tracking methods over active methods is that they do allow “visualization” of the entire length of a device. Since air, cortical bone and flowing blood are also seen in MR images as areas of signal voids, the use of signal void is generally not appropriate for tracking devices used in interventional MR. Another technique of passive tracking utilizes the fact that some materials cause a magnetic susceptibility artifact (either signal enhancement or signal loss) that causes a signal different from the tissue in which they are located. Some catheters braided with metal, some stents and some guide-wires are examples of such devices. One problem with the use of these techniques based on susceptibility artifacts is the fact that those used for localization of the device does not correspond precisely with the size of the device. This makes precise localization difficult.
A number of published reports describe passive catheter visualization schemes based on signal voids or susceptibility-induced artifacts. A principal drawback of these passive techniques is that visualization is dependent on the orientation of the device with respect to the main magnetic field.
Despite recognition and study of various aspects of the problems of visualization of medical devices in therapeutic, especially endovascular, procedures, the prior art has still not produced satisfactory and reliable techniques for visualization and tracking of the entire device in a procedure under MR guidance.